Non-aqueous electrolyte secondary battery

ABSTRACT

A non-aqueous electrolyte secondary battery uses as its positive electrode active material a mixture of a first lithium-containing transition metal oxide containing nickel and manganese as transition metals and having a crystal structure belonging to the space group R3m and a second lithium-containing transition metal oxide containing nickel, cobalt, and manganese as transition metals and having a crystal structure belonging to the space group R3m, or a mixture of the first lithium-containing transition metal oxide and a lithium cobalt oxide. The first lithium-containing transition metal oxide is Li a Ni x Mn y O 2  wherein 1≦a≦1.5, 0.5≦x+y≦1, 0&lt;x&lt;1, and 0&lt;y&lt;1. The second lithium-containing transition metal oxide is Li b Ni p Mn q Co r O 2  wherein 1≦b≦1.5, 0.5≦p+q+r≦1, 0&lt;p&lt;1, 0&lt;q&lt;1, and 0&lt;r&lt;1.

This application is a division of application Ser. No. 11/501,224, filedAug. 9, 2006, which claims priority of Japanese Patent Application Nos.2005-233528 and 2005-278108 filed Aug. 11, 2005, and Sep. 26, 2005,respectively, and which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-aqueous electrolyte secondarybatteries, such as lithium secondary batteries.

2. Description of Related Art

Lithium-ion batteries are light in weight and capable of high power. Forthis reason, lithium-ion batteries in recent years have drawn attentionas batteries for hybrid automobiles. A battery for hybrid automobiles isrequired to show relatively uniform power characteristics over a widerange of charge depth, in addition to high power characteristics. Thepurpose of this requirement is to reduce costs in the system bysimplifying the control algorithm for battery input power.

Lithium-containing nickel-manganese composite oxides have drawnattention as a low-cost positive electrode material because they usenickel and manganese, which are rich in reserve, in comparison withlithium cobalt oxides, which have conventionally been used as a positiveelectrode active material. In particular, the lithium-containingnickel-manganese composite oxides are considered as good candidates fora positive electrode active material in the batteries for hybridautomobiles.

Nevertheless, a non-aqueous electrolyte secondary battery using alithium-containing nickel-manganese oxide as its positive electrodeactive material has the problems of low discharge capacity and poorpower characteristics due to its high resistance.

In view of the above-noted problems, Japanese Published UnexaminedPatent Application No. 2003-92108 proposes the use of alithium-manganese composite oxide having a spinel structure to improvelow-temperature power characteristics of the battery. However, the useof the lithium-manganese composite oxide having a spinel structure,which has a discharge potential of about 4 V (vs. Li/Li⁺), leads to theproblems of insufficient power characteristics and moreover insufficientbattery capacity. In addition, much research has been conducted onlithium-containing nickel-manganese-cobalt oxides as well. Thelithium-containing nickel-manganese-cobalt oxides, however, tend tocause a great increase in resistance at later stages of discharge andare incapable of achieving uniform power characteristics over a widerange of charge depth, which means that the lithium-containingnickel-manganese-cobalt oxides are unsuitable for the batteries forhybrid automobiles.

U.S. Published Patent Application No. 2003/0108793 describes that theuse of a positive electrode material Li(LiNiMn)O₂ in which lithium isarranged at transition metal sites improves discharge capacityconsiderably. Nevertheless, this technique has not yet been satisfactoryin terms of uniform power characteristics.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a non-aqueouselectrolyte secondary battery that is capable of achieving uniform highpower characteristics over a wide range of charge depth and is suitablefor secondary batteries for use in hybrid automobiles or the like.

In order to accomplish the foregoing and other objects, the presentinvention provides a non-aqueous electrolyte secondary batterycomprising: a positive electrode comprising a positive electrode activematerial; a negative electrode comprising a negative electrode activematerial; and a non-aqueous electrolyte having lithium ion conductivity,wherein the positive electrode active material comprises: a mixture of afirst lithium-containing transition metal oxide containing nickel andmanganese as transition metals and having a crystal structure belongingto the space group R3m, the first lithium-containing transition metaloxide being represented by the formula Li_(a)Ni_(x)Mn_(y)O₂ wherein1≦a≦1.5, 0.5≦x+y≦1, 0<x<1, and 0<y<1, and a second lithium-containingtransition metal oxide containing nickel, cobalt, and manganese astransition metals and having a crystal structure belonging to the spacegroup R3m, the second lithium-containing transition metal oxideLi_(b)Ni_(p)Mn_(q)Co_(r)O₂ wherein 1≦b≦1.5, 0.5≦p+q+r≦1, 0<p<1, 0<q<1,and 0<r<1; or a mixture of the first lithium-containing transition metaloxide and a lithium cobalt oxide.

The present invention makes it possible to improve power characteristicsof the battery dramatically and to obtain uniform power characteristicsover a wide range of charge depth, by using a positive electrode activematerial in which the first lithium-containing transition metal oxide ismixed with the second lithium-containing transition metal oxide or withlithium cobalt oxide. Accordingly, when the non-aqueous electrolytesecondary battery of the present invention is used as a secondarybattery for a hybrid automobile, a control algorithm in the hybridautomobile can be simplified and consequently the costs for the systemcan be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the I-V resistances and initial voltages ofthe test cells of Examples 1 to 4 and Comparative Examples 1 and 2 inaccordance with the present invention; and

FIG. 2 is a graph illustrating the relationship between input powerversus mixture ratio of the first lithium-containing transition metaloxide and the second lithium-containing transition metal oxide.

DETAILED DESCRIPTION OF THE INVENTION

The non-aqueous electrolyte secondary battery according to the presentinvention uses, as a positive electrode active material, a mixture of afirst lithium-containing transition metal oxide, which is alithium-containing nickel-manganese oxide Li_(a)Ni_(x)Mn_(y)O₂, wherein1≦a≦1.5, 0.5≦x+y≦1, 0<x<1, and 0<y<1, and a second lithium-containingtransition metal oxide, which is a lithium-containingnickel-manganese-cobalt oxide, Li_(b)Ni_(p)Mn_(q)Co_(r)O₂, wherein1≦b≦1.5, 0.5≦p+q+r≦1, 0<p<1, 0<q<1, and 0<r<1, or a mixture of the firstlithium-containing transition metal oxide and lithium cobalt oxide.Thereby, the power characteristics of the battery are improveddramatically, and at the same time uniform power characteristics can beobtained over a wide range of charge depth.

In the present invention, it is preferable that the weight ratio of themixture of the first lithium-containing transition metal oxide and thesecond lithium-containing transition metal oxide (the firstlithium-containing transition metal oxide:the second lithium-containingtransition metal oxide) and the weight ratio of the mixture of the firstlithium-containing transition metal oxide and lithium cobalt oxide (thefirst lithium-containing transition metal oxide:lithium cobalt oxide) bein the range of 1:9 to 9:1, more preferably in the range of 2:8 to 8:2,and still more preferably in the range of 6:4 to 4:6. Controlling theweight ratios of the mixtures to be within these ranges makes theadvantageous effects of the invention more effective, namely, achievinghigh power characteristics and obtaining uniform power characteristicsover a wide range of charge depth.

More preferable ranges of x and y in the first lithium-containingtransition metal oxide Li_(a)Ni_(x)Mn_(y)O₂ are 0<x≦0.5 and 0.5≦y<1,respectively. It is preferable that the mole ratio (x/y) of Ni/Mn in thefirst lithium-containing transition metal oxide be less than 1, andx+y<1. It is preferable that cobalt be contained in the secondlithium-containing transition metal oxide in a mole ratio of 0.2 orgreater with respect to the total content of the transition metals. Morepreferable ranges of p, q, and r in Li_(b)Ni_(p)Mn_(q)Co_(r)O₂ are0<p≦0.8, 0.5≧r≧0.2, and 0<q≦0.5, respectively.

The first lithium-containing transition metal oxide and the secondlithium-containing transition metal oxide may contain at least oneelement selected from the group consisting of B, Mg, Al, Ti, Cr, V, Nb,Zr, Sn, and Mo at a mole ratio of 0.1 or less, with respect to the totalmoles of the metals other than lithium.

Likewise, the lithium cobalt oxide may contain at least one elementselected from the group consisting of B, Mg, Al, Ti, Cr, V, Nb, Zr, Sn,Mo, W, and P. It is preferable that the content be 0.1 or less, based onthe mole ratio with respect to the total moles of the metals other thanlithium.

In the present invention, a portion of the lithium in each of the firstlithium-containing transition metal oxide and the secondlithium-containing transition metal oxide may be contained at a 3b siteof the transition metals. In this case, it is preferable to set theend-of-charge potential in the initial charging at 4.45 V or higher but4.65 V or less, from the viewpoint of improvement in powercharacteristics. Furthermore, it is preferable that the particle size ofthese oxides be within the range of from 1 μm to 20 μm, and that theseoxides have a BET specific surface area of from 0.1 m²/g to 3 m²/g.

Both the first lithium-containing transition metal oxide and the secondlithium-containing transition metal oxide used in the present inventionhave a crystal structure belonging to the space group R3m. Such acrystal structure may be confirmed by X-ray diffraction analysis. Thelithium cobalt oxide also has a crystal structure belonging to the spacegroup R3m.

The negative electrode active material used in the present invention isnot particularly limited as long as it can be used for non-aqueouselectrolyte batteries, but carbon materials are preferable.

In the present invention, the solute (supporting salt) of thenon-aqueous electrolyte may be any lithium salt that is generally usedas a solute in non-aqueous electrolyte secondary batteries. Examples ofthe lithium salt include LiPF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂) (C₄F₉SO₂), LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃,LiAsF₆, LiClO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, and mixtures thereof. Inaddition to these solutes, the non-aqueous electrolyte may contain alithium salt having an oxalato complex as anion. An example of such alithium salt is lithium-bis(oxalato)borate.

The solvent of the non-aqueous electrolyte used in the present inventionmay be any solvent that has conventionally been used as a solvent for anelectrolyte in non-aqueous electrolyte secondary batteries. Particularlypreferable is a mixed solvent of a cyclic carbonate and a chaincarbonate. Examples of the cyclic carbonate include ethylene carbonate,propylene carbonate, butylene carbonate, and vinylene carbonate.Examples of the chain carbonate include dimethyl carbonate, methyl ethylcarbonate, and diethyl carbonate.

Hereinbelow, the present invention is described in further detail basedon examples thereof. It should be construed, however, that the presentinvention is not limited to the following preferred embodiments butvarious changes and modifications are possible without departing fromthe scope of the invention.

EXAMPLE 1 Preparation of Positive Electrode Active Material

A lithium-containing nickel-manganese oxide, which serves as the firstlithium-containing transition metal oxide in the present invention, wasprepared in the following manner. Li₂CO₃ and (Ni_(0.5)Mn_(0.5))₃O₄ weremixed together at a mole ratio of 1.1:1, and the resultant mixture wasbaked in an air atmosphere at 900° C. for 20 hours. The composition ofthe lithium-containing nickel-manganese oxide thus obtained wasLi_(1.1)Ni_(0.5)Mn_(0.5)O₂.

A lithium-containing nickel-manganese-cobalt oxide, which serves as thesecond lithium-containing transition metal oxide in the presentinvention, was prepared in the following manner. Li₂CO₃ and(Ni_(0.4)Co_(0.3)Mn_(0.3))₃O₄ were mixed together at a mole ratio of1.15:1, and the resultant mixture was baked in an air atmosphere at 900°C. for 20 hours. The composition of the lithium-containingnickel-manganese-cobalt oxide thus obtained wasLi_(1.15)Ni_(0.4)Co_(0.3)Mn_(0.3)O₂.

The lithium-containing nickel-manganese oxide obtained in theabove-described manner had a particle size of 10 μm and a BET specificsurface area of 1.0 m²/g. The lithium-containing nickel-manganese-cobaltoxide obtained in the above-described manner also had a particle size of10 μm and a BET specific surface area of 1.0 m²/g. It was confirmed byX-ray diffraction analysis that both of the materials had a crystalstructure belonging to the space group R3m.

Preparation of Positive Electrode

The first and second lithium-containing transition metal oxides thusprepared were mixed together at a weight ratio of 8:2 (the firstlithium-containing transition metal oxide:the second lithium-containingtransition metal oxide). The resultant mixture was mixed with a carbonmaterial as a conductive agent and an N-methyl-pyrrolidone solution inwhich polyvinylidene fluoride as a binder agent was dissolved, so thatthe weight ratio of the active material, the conductive agent, and thebinder agent were 90:5:5 to prepare a positive electrode slurry. Theslurry thus prepared was applied onto an aluminum foil serving as acurrent collector and then dried. Thereafter, the aluminum foil coatedwith the positive electrode slurry was pressure-rolled with pressurerollers, and a current collector tab was attached thereto. Thus, apositive electrode was prepared.

Preparation of Electrolyte Solution

LiPF₆ as a solute was dissolved at a concentration of 1 mole/liter intoa mixed solvent of 3:7 volume ratio of ethylene carbonate (EC) anddiethyl carbonate (DEC) to thus prepare an electrolyte solution.

Preparation of Three-Electrode Beaker Cell

The positive electrode prepared in the above-described manner was usedas a working electrode, and metallic lithium was used for a counterelectrode and a reference electrode. The electrolyte solution preparedin the above-described manner was filled into a container into whichthree electrodes were arranged. Thus, a three-electrode beaker cell Alwas prepared.

EXAMPLE 2

A test cell A2 was prepared in the same manner as in Example 1, exceptthat the mixture ratio of the first lithium-containing transition metaloxide and the second lithium-containing transition metal oxide was 6:4(weight ratio).

EXAMPLE 3

A test cell A3 was prepared in the same manner as in Example 1, exceptthat the mixture ratio of the first lithium-containing transition metaloxide and the second lithium-containing transition metal oxide was 4:6(weight ratio).

EXAMPLE 4

A test cell A4 was prepared in the same manner as in Example 1, exceptthat the mixture ratio of the first lithium-containing transition metaloxide and the second lithium-containing transition metal oxide was 2:8(weight ratio).

COMPARATIVE EXAMPLE 1

A test cell B was prepared in the same manner as in Example 1, exceptthat the first lithium-containing transition metal oxide alone was usedas the positive electrode active material.

COMPARATIVE EXAMPLE 2

A test cell C was prepared in the same manner as in Example 1, exceptthat the second lithium-containing transition metal oxide alone was usedas the positive electrode active material.

COMPARATIVE EXAMPLE 3

A test cell D was prepared in the same manner as in Example 1, exceptthat the positive electrode active material was an 8:2 weight ratiomixture of the first lithium-containing transition metal oxide and alithium manganese oxide (Li_(1.1)Mn_(1.9)O₄) having a spinel structure.

The test cells prepared in the manners described in Examples 1 to 4 andComparative Examples 1 to 3 were subjected to the followingcharge-discharge test and I-V resistance measurement test.

Charge-Discharge Test

Each of the cells was charged at 1 mA to 4300 mV (vs. Li/Li⁺) at roomtemperature, then rested for 10 minutes, and then discharged at 1 mA to2000 mV (vs. Li/Li⁺), to thus calculate the rated discharge capacity.

I-V Resistance Measurement Test

The cells were discharged to 2000 mV (vs. Li/Li⁺) under thejust-described charge-discharge test condition, and thereafter chargedat 1 mA to 10% and 50% of the rated discharge capacity, and thefollowing tests were carried out.

(1) 5 mA charge (10 seconds)→rest (10 minutes)→5 mA discharge (10seconds)→rest (10 minutes)

(2) 10 mA charge (10 seconds)→rest (10 minutes)→10 mA discharge (10seconds)→rest (10 minutes)

(3) 20 mA charge (10 seconds)→rest (10 minutes)→20 mA discharge (10seconds)→rest (10 minutes)

The charge-discharge tests (1) to (3) were carried out in that order atroom temperature. The highest potential reached in each charging wasmeasured, and from the gradient of the potential values with respect tothe current values, I-V resistances were calculated. From The I-Vresistances obtained and the initial voltage Vo at the start of theabove test (1), input power values were calculated using the followingequation:

Input power (W)=(4300−Vo)/I-V resistance×4300

The rated discharge capacity, the initial voltage Vo, the I-Vresistance, and the input power of each of the test cells are shown inTable 1 (input power characteristics at 50% SOC) and Table 2 (inputpower characteristics at 10% SOC).

TABLE 1 Li_(1.1)Ni_(0.5)Mn_(0.5)O₂ Rated discharge Initial voltage I-VInput mixture ratio capacity Vo (mV vs. resistance power Cell (wt. %)(mAh/g) Li/Li⁺) (mΩ) (W) Ex. 1 A1 80 124 3902 3845 444 Ex. 2 A2 60 1333881 2914 617 Ex. 3 A3 40 139 3866 2492 748 Ex. 4 A4 20 145 3847 2085933 Comp. B 100 116 3927 6655 240 Ex. 1 Comp. C 0 149 3830 2082 969 Ex.2 Comp. D 80 (mixture with 133 3930 2333 681 Ex. 3 lithium manganeseoxide)

TABLE 2 Difference between Proportion of I-V Input Input power at 50%Li_(1.1)Ni_(0.5)Mn_(0.5)O₂ resistance power SOC - Input power Cell (wt.%) (mΩ) (W) at 10% SOC Ex. 1 A1 80 4424 527 83 Ex. 2 A2 60 3614 698 81Ex. 3 A3 40 3800 634 114 Ex. 4 A4 20 3783 606 327 Comp. B 100 7607 29252 Ex. 1 Comp. C 0 4007 585 384 Ex. 2 Comp. D 80 (mixture with 6670 389292 Ex. 3 lithium manganese oxide)

FIG. 1 shows the I-V resistances and the initial voltages of the testcells of Examples 1 to 4 and Comparative Examples 1 and 2. FIG. 2 showsthe relationship between input power and the mixture ratios of the firstlithium-containing transition metal oxide Li_(1.1)Ni_(0.5)Mn_(0.5)O₂. Asclearly seen from FIGS. 1 and 2, adding the second lithium-containingtransition metal oxide to the first lithium-containing transition metaloxide according to the present invention caused the I-V resistance andthe input power to differ from those expected from the weighted averageof the mixture ratios, exceptionally improving the I-V resistance andinput power. In addition, by adding the second lithium-containingtransition metal oxide, the initial voltage Vo lowered, improving theinput power characteristics. Comparative Example 3, in which the lithiummanganese oxide having a spinel structure (Li_(1.1)Mn_(1.9)O₄) wasadded, did not yield sufficient input power characteristics because ofthe increase of initial voltage Vo originating from its high dischargepotential.

In addition, as shown in Table 2, Comparative Example 2, which used thelithium-containing nickel-manganese-cobalt oxide alone, showed thegreatest input power difference between the input power at 50% SOC andthat at 10% SOC. In contrast, Examples 1 to 4, each of which used amixture of a lithium-containing nickel-manganese oxide and alithium-containing nickel-manganese-cobalt oxide according to thepresent invention, showed smaller differences between the input power at50% SOC and the input power at 10% SOC than Comparative Example 2, andmoreover, they yielded relatively high input power. Thus, it will beunderstood that the present invention makes it possible to obtainuniform and high input power over a wide range of charge depth.

Although the details of workings of the above-described advantageouseffect achieved by the present invention have not yet been clearlyunderstood, it is believed that adding a lithium-containingnickel-manganese-cobalt oxide to a lithium-containing nickel-manganeseoxide improves the electrochemical activity of the lithium-containingnickel-manganese oxide, and moreover, reduces the dependence of thepower characteristics on the depth of charge and discharge.

EXAMPLE 5

A test cell A5 was prepared in the same manner as in Example 1, exceptfor the following. Li₂Co₃ and (Ni_(0.1)Mn_(0.6))₃O₄ were mixed togetherat a mole ratio of 1.3:0.7, and the resultant mixture was baked in anair atmosphere at 1000° C. for 20 hours, to thus prepare alithium-containing nickel-manganese oxide Li_(1.3)Ni_(0.1)Mn_(0.6)O₂,which was used as the first positive electrode active material. Thefirst positive electrode active material thus prepared was mixed withthe second positive electrode active materialLi_(1.15)Ni_(0.4)Co_(0.3)Mn_(0.3)O₂ at a weight ratio of 1:1.

EXAMPLE 6

A test cell A6 was prepared in the same manner as in Example 5, exceptthat a lithium cobalt oxide containing other elements,LiCo_(0.99)Zr_(0.005)Mg_(0.005)O₂, was used as the second positiveelectrode active material.

COMPARATIVE EXAMPLE 4

A test cell E was prepared in the same manner as in Example 1, exceptthat Li_(1.3)Ni_(0.1)Mn_(0.6)O₂ alone was used as the positive electrodeactive material.

COMPARATIVE EXAMPLE 5

A test cell F was prepared in the same manner as in Example 1, exceptthat a lithium cobalt oxide containing other elementsLiCo_(0.99)Zr_(0.005)Mg_(0.005)O₂ alone was used as the positiveelectrode active material.

Charge-Discharge Test

Each of the cells was charged at 1 mA to 4600 mV (vs. Li/Li⁺) at roomtemperature, then rested for 10 minutes, and then discharged at 1 mA to2000 mV (vs. Li/Li⁺), to thus calculate the discharge capacity.

I-V Resistance Measurement Test

The cells were discharged to 2000 mV (vs. Li/Li⁺) under thejust-described charge-discharge test condition, and thereafter chargedat 1 mA to 30% and 70% of the discharge capacity, and the followingtests were carried out.

(1) 5 mA charge (10 seconds)→rest (10 minutes)→5 mA discharge (10seconds)→rest (10 minutes)

(2) 10 mA charge (10 seconds)→rest (10 minutes)→10 mA discharge (10seconds)→rest (10 minutes)

(3) 20 mA charge (10 seconds)→rest (10 minutes)→20 mA discharge (10seconds)→rest (10 minutes)

The charge-discharge tests (1) to (3) were carried out in that order atroom temperature. The highest potential reached in each charging wasmeasured, and from the gradient of the potential values with respect tothe current values, I-V resistances were calculated. From The I-Vresistances obtained and the initial voltage Vo at the start of theabove test (1), input power values were calculated using the followingequation.

Input power (W)=(4300−Vo)/I-V resistance×4300

The input power and input power difference for each of the cells areshown in Table 3.

TABLE 3 Input Input Input power at power at power at Positive electrodeSOC 70% SOC 30% SOC 30%-70% Cell active material (W) (W) (W) Ex. 5 A5Li_(1.3)Ni_(0.1)Mn_(0.6)O₂ + 420 463 43 Li_(1.15)Ni_(0.4)CO_(0.3)O₂ 1:1mixture Ex. 6 A6 Li_(1.3)Ni_(0.1)Mn_(0.6)O₂ + 167 174 6.8LiCo_(0.09)Zr_(0.005)Mg_(0.005)O₂ 1:1 mixture Comp. ELi_(1.3)Ni_(0.1)Mn_(0.6)O₂ 110 230 120 Ex. 4 alone Comp. FLiCo_(0.09)Zr_(0.005)Mg_(0.005)O₂ 408 658 250 Ex. 5 alone

The test results shown in Table 3 clearly demonstrate that mixingLi_(1.3)Ni_(0.1)Mn_(0.6)O₂ with eitherLi_(1.15)Ni_(0.4)Co_(0.3)Mn_(0.3)O₂ or LiCo_(0.99)Zr_(0.005)Mg_(0.005)O₂makes it possible to reduce variations in input power with respect tochanges in SOC considerably. Thus, it is possible to fabricate a batterythat shows little dependence of power on the depth of charge anddischarge.

Although the details of workings of the above-described advantageouseffect achieved by the present invention have not yet been clearlyunderstood, it is believed that the effects of improving theelectrochemical activity of the lithium-containing nickel-manganeseoxide and reducing the dependence of power on charge depth, which isachieved by the addition of another type of positive electrode activematerial, emerges more evidently because the positive electrode materialcomprising a lithium-containing nickel-manganese oxide having a largeproportion of manganese shows a low electron conductivity.

Only selected embodiments have been chosen to illustrate the presentinvention. To those skilled in the art, however, it will be apparentfrom the foregoing disclosure that various changes and modifications canbe made herein without departing from the scope of the invention asdefined in the appended claims. Furthermore, the foregoing descriptionof the embodiments according to the present invention is provided forillustration only, and not for limiting the invention as defined by theappended claims and their equivalents.

1. A non-aqueous electrolyte secondary battery comprising: a positiveelectrode comprising a positive electrode active material; a negativeelectrode comprising a negative electrode active material; and anon-aqueous electrolyte having lithium ion conductivity, wherein thepositive electrode active material comprises a mixture of a firstlithium-containing transition metal oxide containing nickel andmanganese as transition metals and having a crystal structure belongingto the space group R3m, the first lithium-containing transition metaloxide being represented by the formula Li_(a)Ni_(x)Mn_(y)O₂ wherein1≦a≦1.5, 0.5≦x+y≦1, 0<x<1, and 0<y<1, and a second lithium-containingtransition metal oxide containing nickel, cobalt, and manganese astransition metals and having a crystal structure belonging to the spacegroup R3m, the second lithium-containing transition metal oxide beingrepresented by the formula Li_(b)Ni_(p)Mn_(q)Co_(r)O₂ wherein 1≦b≦1.5,0.5≦p+q+r≦1, 0<p<1, 0<q<1, and 0<r<1.
 2. The non-aqueous electrolytesecondary battery according to claim 1, wherein the weight ratio of themixture of the first lithium-containing transition metal oxide and thesecond lithium-containing transition metal oxide (the firstlithium-containing transition metal oxide:the second lithium-containingtransition metal oxide) is in the range of 2:8 to 8:2.